Leaves of all trees contain chlorophyll, a green pigment that has the unusual capability to capture light energy and (with the help of other components in the leaf) to convert that energy into a chemical form, such as sugar. Many leaves contain other pigments as well, and while these pigments can't photosynthesize as chlorophyll can, some of them are able to transfer the light energy they capture to the chlorophyll. Some of these "accessory" pigments are yellow, orange, or red and are called carotenoids because they belong to the same group of compounds as beta-carotene, the pigment that gives carrots their orange color (and margarine its yellow).
In the autumn, when deciduous leaves begin to get old, the leaf is able to break down some of the expensive pigments it has produced (such as chlorophyll) and absorb parts of them back into the stems for other uses. When the green color of chlorophyll is gone, the other colors are unmasked.
You can see these colors when the leaves are still green if you separate the pigments by a process called chromatography. If you have ever watched water-soluble ink smear on paper when it gets wet, you have seen chromatography in action. Separating the pigments from leaves is a little harder, because they are often enclosed in membranes within the cells of a leaf. But if you have some filter paper (try using a white coffee filter) you could try to express some of the pigments onto it by placing the leaf on the filter and then rolling a quarter across the leaf several times to make a line of pigments on the paper. Then dip one end of the paper in rubbing alcohol, and you might be able to see some of the other colors in the leaf separate from the green chlorophyll.
Some pigments in leaves--such as the reddish-purple in rhubarb or red cabbage--are not involved in photosynthesis at all. Perhaps they help protect the plant against too much sunlight? These compounds are held in other places in the cells of the leaf, and many of them are water-soluble, so if you cook the leaf or grind it in a blender, you will release this reddish pigment in the water.
Patricia Hauslein is a member of the department of biological sciences at St. Cloud State University in St. Cloud, Minn. She offers this primer:
"I don't believe it! Here it is only the third week of August and already that tree on 9th Street is changing." I've been hearing that a lot of comments like this last week. Every year when we see the trees beginning to change color here in Central Minnesota we start to believe we must be heading for an early winter. I think if we paid attention, however, we would see that the same tree starts to change colors about the same time every year.
So what is it that causes the leaves to take on their fall colors? What has changed in the last few weeks? It has been cooler and wetter here than usual, but last year at this time it was hot and dry and that same tree still changed colors. The thing that seems to happen at the same time every year is the shortening of the day (or, more accurately, the amount that the daylight diminishes as the summer wanes). In fact, it is the length of the day, called the photoperiod, that triggers a mechanism in the tree to begin the process of dropping the leaves before winter. This process of shedding leaves is necessary for the tree, but it has the additional benefit to us of an explosion of color which we get to enjoy for a few short weeks in the fall.
To fully answer the question, "Why do leaves change color and why those colors?", it would be best to break it down into a couple of other questions. We have already know when leaves change color: in the fall, to get the tree ready for winter. We can then ask, what do leaves do for the tree in the summer that the tree doesn't need to do during the winter? The leaves are the manufacturing plant of the tree. They capture the sun's light energy and use it to transform carbon dioxide from the air and rainwater into sugars. These sugars sustain the tree; any extra that is made allows the tree to grow larger. This process, called photosynthesis (which means "light put-together"), also pulls water up the tree, where it than evaporates from the surface of the leaves. If the leaves stayed on all winter, the tree would continue to lose water without much chance to re-supply it once the ground had frozen. So the leaves drop off to conserve water.
Now we can ask the next question. "Why do the leaves change color before they drop?" Remember that the tree is able to make sugar by capturing energy from sunlight. There are pigments in leaves which absorb that solar energy and send it off to sugar production. The most abundant pigment is chlorophyll, which we see as the green color of summer leaves. But the light from the sun is basically what is called white light. That means that sunlight is actually a combination of all colors. (Remember ROY G BIV, the rainbow colors?: red, orange, yellow, green, blue, indigo, and violet.) This combination of all colors is striking the leaves, but only certain colors are actually absorbed by the leaves; the others are reflected. We see reflected light as the color of an object. So we know that green light is reflected off the leaves. That means that the leaves use colors other than green to work in the production of sugars. In fact, chlorophyll absorbs mostly blue and some orange light.
One thing we have learned about nature is that it does not waste anything. Although chlorophyll only absorbs blue and orange light, other pigments in the leaves absorb the other colors. Some of those other pigments are called carotenoids. They absorb green light and reflect orange. Carrots have a lot of carotenoids. During the summer, there is so much chlorophyll in the leaves we simply cannot see the other pigments. But as the daylight shortens, the tree does not make as much chlorophyll. As the chlorophyll starts to fade away, we are able to see the other colors (pigments) in the leaf, mostly yellow ones.
Where I live it is the aspens and poplars that turn bright yellow. The red and oranges are mostly seen in the sugar maples. Maples turn red because when the leaf-dropping process begins in these trees, some of the sugar that the leaves made remains trapped in the leaves. In this case, the color is dominated by a third type of light-absorbing pigment, one that reacts with the sugars and makes the red and orange color we see. The brighter the days are during fall, the more sugar gets trapped in the leaves and the more brilliant are the colors of the sugar maples.
You might want to try some experiments. Cover a green leaf, still on a tree, with black paper. When the leaves around it have changed, uncover the leaf and see what color it is. Black paper absorbs all the light and turns it into heat...though I wonder what would happen if you covered the green leaf with aluminum foil instead. I wonder what an aspen leaf would do compared to a maple, or an oak. I wonder what would happen if you took the leaf off the tree. I wonder...excuse me, I have some experimenting of my own to do.